I also make my own extensions using the crimpling tool and purchased components. It takes me about 5 minutes to complete an extension, the extensions are sized exactly the way I want them including Y harness's.
I dont like soldier unless absolutely necessary for a few reasons:

1: the soldier stiffens the wire and with vibration, the wire can fail at the point the solder ends
2: shrink tubing works well but the stiff soldier can wear right though the tubing.
3: you must be very cautious of a cold joint... ie where the soldier did not really penitrate or the two wires moved while cooling. This can result in a high resistance joint and is hard to detect, especially for a novice.
4: Excessive heat will damage the insulation on the wire so the soldering tool must be just right. A too large iron or gun heats so fast the damage is done in a split second. I have soldiered for years and can control the heat pretty well but I still overdo it sometimes.

Best regards,
Randy